Surface micromachining by sacrificial aluminium etching

Sacrificial aluminium etching enables micromechanical structures integrated with circuitry to be fabricated using standard IC processes followed by simple post-processing. In this paper, the etching characteristics of CMOS aluminium in four etch solutions are reported. The solutions are (A) a commercially available aluminium etchant, (B) Krumm etch, (C) diluted hydrochloric acid, and (D) diluted hydrochloric acid with hydrogen peroxide. The etching of narrow channels is studied as a function of time and temperature. Initially, the etching process is reaction-rate controlled and then crosses over to a diffusion-controlled regime with reduced etch rate. Underetching distances larger than 250 mu m are readily achieved with etchants 'A','B', and 'D'. The commercially available aluminium etchant has a low initial underetch rate of 68 mu m h(-1) at 50 degrees C but offers best control. The initial etch rate of hydrochloric acid with hydrogen peroxide is 500 mu m h(-1) at 30 degrees C. However, irregular etch fronts are obtained. Reliable protection of aluminium pads against etchants 'A','B', and 'D' is guaranteed by Shipley's photoresist S1828 spun at 3000 rpm and hardbaked at 140 degrees C.